1. Field of the Invention
This invention pertains, in general, to making analog form playbacks from digitally recorded data (e.g., seismic data) which has been digitized from wide dynamic amplitude range analog for signals.
2. Description of the Prior Art
In seismic exploration work each acoustically driven geophone generates wide dynamic amplitude range signals in analog form. When such signals are processed through a digital seismic recording system there is produced a high fidelity record in digital form covering the dynamic range of amplitudes of the seismic signals. The reason that the digital form record is referred to herein as a high fidelity record is because the signal amplitudes are recorded accurately throughout their wide dynamic range; e.g., many binary digit or bit positions are used to record the highest signal amplitude as well as the lowest where the range (i.e., the ratio of the highest signal amplitude to the lowest signal amplitude) may be of the order of 10.sup.6.
The present invention provides a method and apparatus for making analog form oscillograms or wiggle traces from the recorded digital data. The oscillograms are of relatively lower fidelity than the aforementioned digitally recorded data. Although the oscillograms are of relatively lower fidelity serious distortions are nevertheless not introduced in reconverting the digital data to analog data for the purpose of making compressed amplitude range oscillograms. The recordation in digital form of wide dynamic amplitude range analog signals initially generated by geophones is disclosed in, among other, the following: U.S. Pat. No. 3,241,100 issued Mar. 15, 1966 in behalf of R. J. Loofbourrow and entitled "Digital Seismic Recording System"; U.S. Pat. No. 3,264,574 issued Aug. 2, 1966 in behalf of R. J. Loofbourrow and entitled "Amplifier System"; U.S. Pat. No. 3,603,972 issued Sept. 7, 1971 to James R. Vanderford and entitled "Amplifier System"; U.S. Pat. No. 3,685,046 issued Aug. 15, 1972 to Donald L. Howlett and entitled "Seismic Playback/Monitor System" and U.S. Pat. No. 3,872,465 issued Mar. 18, 1975 to Robert J. Loofbourrow and entitled "Seismic Playback/Monitor System".
As is disclosed in the patents hereinbefore identified the problem solved is the problem of accurately recording seismic data which in analog form has a dynamic range of amplitude which is extremely wide. For example, a typical analog signal level for a reflection seismic record runs from several volts of amplitude at its maximum at the early shock portion of the record to less than a single microvolt at the end of the seismic record when low amplitude seismic disturbances are detected. In general the aforementioned patents disclose systems for converting the wide dynamic amplitude range analog signal to digital form. When converted to digital form occupying a relatively large number of binary digit or bit positions the full dynamic amplitude range of the analog signals initially generated by a geophone is preserved in recorded form; e.g., on magnetic tape. Advantageously the magnetically recorded digital data may afterward be delivered to a computer for further processing. Some methods and some purposes for which such digital data is subsequently processed in a computer are disclosed in an article "Tools for Tomorrow's Geophysics" by Milton D. Vobrin and Stanley H. Ward, published in the Journal "Geophysical Prospecting" Vol. X at pp. 433-452 (1962).
In the aforementioned Vanderford and Howlett patents there is disclosed a system wherein portions of an analog signal are converted to digital word wherein each digital word occupies a number of binary digit or bit positions. Also each such digital word is recorded in floating point form. The floating point form of notation allows greater flexibility of operation and easier handling of numbers which differ greatly in magnitude from each other. See for example the textbook "Digital Computer Primer" by E. M. McCormick, 1959, published by McGraw-Hill Book Company, Inc. beginning at page 152. In the system disclosed in the Vanderford and Howlett patents hereinbefore identified, a floating point digital word or number in the form of a mantissa or argument and an exponent is recorded on a suitable storage medium such as magnetic tape. The floating point digital word represents the instantaneous absolute seismic voltage amplitude as it is introduced to a floating point amplifier system. The dynamic range of the floating point word may be in excess of 200 db if necessary to cover the dynamic range of input signals (equivalent to a 36 binary digit or bit digital number or word).
As a specific example the floating point word or number as set forth in conventional algebraic form is as follows: EQU V.sub.in = .+-. AG .sup.-E
wherein V.sub.in represents the absolute magnitude or amplitude of the floating point word; A represents the mantissa or argument portion of the word; G represents the radix or base of the number system used (G = 10 in the decimal system or G = 8 in the octal system); and E represents the exponent.
As is suggested in the aforementioned patents the floating point digital word is in the form: EQU Q = .+-. A8 .sup.-E
wherein Q represents the absolute magnitude of the amplitude of the input signal delivered to an arrangement of amplifiers each of which has a gain of 8 and hence, the base G in equation 1 becomes 8 in equation 2; the mantissa A represents the output amplitude of a particular amplifier in the aforesaid arrangement; and E, the exponent represents the number of amplifier stages of gain of 8 through which the aforesaid input signal has been processed.
In order to record the floating point digital word of equation 2 in a binary register with for example, 144 db of dynamic range and with 14 bit accuracy 18 bit position would be required; wherein the mantissa A is represented in binary form and where the mantissa A is represented in binary form and where the exponent E is also represented in binary form. Of the 18 bits required, one bit represents the sign allowing for bi-polar input-output capabilities; 14 bits represent the mantissa A; and three bits represent the exponent E.
Although there are many advantages to recording seismic signals in digital form there still remains the need to make available to a seismic prospector a visible display or recording of the seismic data or portions of it. Conventionally the visible record is an oscillogram or wiggle trace as it is often called by seismic prospectors. Often it is desirable for a seismic prospector and a seismic field crew in a remote location from a main data processing center to take a quick look at a portion of the seismic data from time to time for example, a seismic prospector may wish to make some interpretations with respect to the oscillogram in order to coordinate such data with geological data.
The invention hereinafter disclosed and illustrated in the accompanying drawings, is particularly concerned with converting the recorded seismic data to the familiar wiggle trace form on recording paper. The recording paper allows about 40 db dynamic amplitude range while the digital floating point word may have a dynamic range of 156 db or more. Hence, in converting from digital form to a practical analog form, selective compression of the various amplitudes must occur. In such a conversion distortion is necessarily introduced. However, in accordance with the methodology and apparatus embodying the subject invention such distortion is minimized and as a result there is provided analog form data in the form of oscillogram or wiggle traces which provide useful information to seismic prospectors among others.